The first standard for C was published by ANSI. Although this document was subsequently adopted by International Organization for Standardization (ISO) and subsequent revisions published by ISO have been adopted by ANSI, the name ANSI C (rather than ISO C) is still more widely used. While some software developers use the term ISO C, others are standards body–neutral and use Standard C.

In 1983, the American National Standards Institute formed a committee, X3J11, to establish a standard specification of C. After a long and arduous process[citation needed], the standard was completed in 1989 and ratified as ANSI X3.159-1989 "Programming Language C." This version of the language is often referred to as "ANSI C". Later on sometimes the label "C89" is used to distinguish it from C99 but using the same labelling method.

The same standard as C89 was ratified by the International Organization for Standardization as ISO/IEC 9899:1990, with only formatting changes,[1] which is sometimes referred to as C90. Therefore, the terms "C89" and "C90" refer to essentially the same language.

In 1995 the ISO published an extension, called Amendment 1, for the ANSI-C standard. Its full name finally was ISO/IEC 9899/AMD1:1995 or nicknamed C95. Aside to error correction there were further changes to the language capabilities,[4][5] such as:

Improvements to the support for multi byte and wide character char sets in the standard library.

ANSI C is now supported by almost all the widely used compilers. Most of the C code being written nowadays is based on ANSI C. Any program written only in standard C and without any hardware dependent assumptions is virtually guaranteed to compile correctly on any platform with a conforming C implementation. Without such precautions, most programs may compile only on a certain platform or with a particular compiler, due, for example, to the use of non-standard libraries, such as GUI libraries, or to the reliance on compiler- or platform-specific attributes such as the exact size of certain data types and byte endianness.

In the above example, a prototype is used in a function declaration for ANSI compliant implementations, while an obsolescent non-prototype declaration is used otherwise. Those are still ANSI-compliant as of C99. Note how this code checks both definition and evaluation: this is because some implementations may set __STDC__ to zero to indicate non-ANSI compliance.